The greening disease Huanglongbiong which threatens citrus trees worldwide can be detected from the volatile biomarkers given off by the pathogen, even though there are no visible symptoms of infection, say researchers from California and Florida, two states under threat. This provides a timely way to monitor infection and identify trees to be cut down, because the disease can go undetected for years. However, during this time it can be transmitted from tree to tree via insects. Leaves will start to turn yellow before dropping off prematurely, twigs and roots will die back and, ultimately, the plants will die.

The disease is caused by the bacterium Candidatus liberibacter but, luckily, its presence leads to the emission of a series of volatile compounds which are specific to it. The research team developed a portable system based on a gas chromatograph coupled to a differential mobility spectrometer and tested it on infected trees which showed no signs of infection as well as trees in outdoor orchards, as they discussed in Analytical Chemistry.

A custom-made trap drew in the volatile compounds and the GC/DMS profiles were classified by a multiway partial least squares analysis to reveal distinctive patterns for diseased plants. The accuracy of the model was about 90% throughout the year as a whole but close to 100% during summer, presumably due to the smaller influence of daily weather conditions. Accuracy was poorer in the spring, due to the effects of volatiles from the blossom.

The top five discriminating volatile compounds were different depending on whether the plant was healthy, asymptomatic or mildly or severely infected, giving good discrimination not only of infected vs. healthy plants but also of the degree of infection.

Although the method is new and the testing device is a prototype, the method shows promise due to its portability, speed and non-invasive nature. "The potential value of this early-detection solution to the citrus industry is tremendous," say the team. "By flagging infected trees early at the asymptomatic stage, eradication would be both more effective and minimized, since it would take place well before other trees become infected and enter the latent period. This would interrupt the deadly infestation cycle at the source, significantly reduce the heavy costs of losing trees and citrus produce."